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| United States Patent |
5,105,291
|
|
Matsumoto
, et al.
|
April 14, 1992
|
Liquid crystal display cell with electrodes of substantially amorphous
metal oxide having low resistivity
Abstract
A liquid crystal display cell comprising a pair of flexible polymer film
substrates having a transparent electrode disposed on the surface thereof
and a liquid crystal material sandwiched therebetween. The transparent
electrode is a metal oxide film which has a surface resistivity of not
greater than 100 ohm per square, a bending property of not greater than
2.0, a curling degree, H, of 0<H<5 mm and a transmittance to light of 550
nm of not less than 75%. Further, this metal oxide is substantially
amorphous, and is formed by a DC sputtering method.
| Inventors:
|
Matsumoto; Fuyuhiko (Kawasaki, JP);
Tani; Masato (Kawasaki, JP);
Enomoto; Takamichi (Kanagawa, JP)
|
| Assignee:
|
Ricoh Company, Ltd. (Tokyo, JP)
|
| Appl. No.:
|
607032 |
| Filed:
|
October 31, 1990 |
Foreign Application Priority Data
| Current U.S. Class: |
349/139; 174/257; 204/192.29; 427/126.3 |
| Intern'l Class: |
G02F 001/133; H01B 001/00; C23C 014/00; B05D 005/12 |
| Field of Search: |
350/336
174/257
204/192.29
427/126.3
359/87 US
|
References Cited
U.S. Patent Documents
| 4121238 | Oct., 1987 | Bachmann et al. | 357/17.
|
| 4792210 | Dec., 1988 | Maurice | 350/334.
|
| 4835061 | May., 1989 | Ohta et al. | 174/257.
|
| 5013416 | May., 1991 | Murayama et al. | 204/192.
|
Other References
Vossen, J. L., "Control of Film Properties by rf-Sputter Techniques",
Journal of Vacuum Science and Technology, vol. 8, No. 5, pp. 512-529,
1971.
Fraser et al., "High Conductivity, Transparent Films of Sputtered
In.sub.2-x Sn.sub.x O.sub.3-y ", J. Electrochemical Soc.: Solid-State
Science and Technology, vol. 119, No. 10, pp. 1368-1374, 10/1972.
Smith et al., "Effects of Heat-Treatment on Indium-Tin Oxide Films", J.
Electrochemical Soc: Solid-State Science and Technology, vol. 128, No. 11,
pp. 2388-2394, 11/81.
Ridge et al., "Composition Control in Conducting Oxide Thin Films", Thin
Solid Films, 96, pp. 121-127, 10/82.
|
Primary Examiner: Miller; Stanley D.
Assistant Examiner: Trice; Ron
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt
Claims
What is claimed is:
1. A liquid crystal display cell comprising a pair of flexible polymer film
substrates having a transparent electrode disposed on the surface thereof
and a liquid crystal layer sandwiched therebetween, said transparent
electrode being a metal oxide film which has an amorphous portion of not
less than 50% in the entire metal oxide film, a surface resistivity of 10
to 80 ohm per square, a transmittance to light of 550 nm of not less than
75%, and a bending property of not greater than 2.0.
2. A liquid crystal display cell according to claim 1, wherein the
transparent electrode has a curling degree, H, of 0<H<5 mm and an etching
time of not greater than 90 sec.
3. A liquid crystal display cell according to claim 1, wherein the
transparent electrode is formed by a DC sputtering method.
4. A liquid crystal display cell according to claim 3, wherein the DC
sputtering method is carried out under film-forming conditions of a mixing
ratio (partial pressure ratio) between a reaction gas and a carrier gas of
5/1000-10/1000, a vacuum degree of 10.sup.-3 -10.sup.-4 Torr, a
film-forming rate of 5-13 .ANG./sec, a sputtering voltage of 200-500 V and
a substrate temperature of not higher than 120.degree. C.
5. A liquid crystal display cell according to claim 1, wherein the
thickness of the transparent electrode is from 500 to 2000 .ANG..
6. A liquid crystal display cell according to claim 1, wherein said metal
oxide film is selected from the group consisting of ITO, indium oxide, tin
oxide, zinc oxide or antimony oxide.
7. A liquid crystal display cell comprising a pair of flexible polymer film
substrates having a transparent electrode disposed on the surface thereof
and a liquid crystal layer sandwiched therebetween, said transparent
electrode being a metal oxide film which has an amorphous portion of not
less than 70% in the entire metal oxide film, a surface resistivity of 10
to 80 ohm per square, a transmittance to light of 550 nm of not less than
80%, a bending property of not greater than 1.5, a curling degree, H, of
0<H<5 mm and an etching time of not greater than 90 sec.
8. A liquid crystal display cell according to claim 7, wherein the
transparent electrode is formed by a DC sputtering method.
9. A liquid crystal display cell according to claim 8, wherein the DC
sputtering method is carried out under film-forming conditions of a mixing
ratio (partial pressure ratio) between a reaction gas and a carrier gas of
5/1000-10/1000, a vacuum degree of 10.sup.-3 -10.sup.-4 Torr, a
film-forming rate of 5-13 .ANG./sec, a sputtering voltage of 200-500 V and
a substrate temperature of not higher than 120.degree. C.
10. A liquid crystal display cell according to claim 7, wherein the
thickness of the transparent electrode is from 500 to 2000 .ANG..
11. A liquid crystal display cell according to claim 7, wherein said metal
oxide film is selected from the group consisting of ITO, indium oxide, tin
oxide, zinc oxide or antimony oxide.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display cell and, more
particularly, it relates to a liquid crystal display cell in which a
liquid crystal material is sandwiched between a pair of flexible polymer
film substrates having a specific transparent electrode disposed on the
surface thereof.
In liquid crystal display cells, those having transparent conductive films
(transparent electrodes) formed on the surface of a substrate made of a
high melting material such as glass or quartz, have often been used
generally. In the case of using such a high melting substrate, since the
transparent electrode film can be formed at a relatively high temperature
(higher than 150.degree. C.), the thus formed transparent conductive film
is highly crystalline and has great toughness. In addition, upon
manufacturing a liquid crystal cell, since the substrate is rigid, the
substrate is not bent during the manufacturing steps and as a result, no
cracks are formed in the transparent electrode.
Along with a recent trend in the liquid crystal display cell of reducing
the weight and the thickness, it has been attempted to replace the high
melting substrate such as glass with a synthetic resin substrate (in
particular, a flexible polymer film substrate). For instance, it has been
proposed a liquid crystal display cell produced by using a synthetic resin
film as a substrate and forming a transparent conductive film made of a
metal oxide such as ITO, indium oxide, tin oxide or antimony oxide on the
flexible synthetic resin substrate (refer, for example, to Japanese Patent
Publication (KOKOKU) No. 62-3929, Japanese Patent Application Laid-Open
(KOKAI) Nos. 62-187324 and 63-18327).
In the liquid crystal display cell using the flexible polymer film as the
substrate, however, since the substrate is flexible, the resistivity is
increased due to the occurrence of cracks in the transparent electrode
during the manufacturing steps of the liquid crystal display cell, or
cracks arise in the transparent electrode by pressurizing or heating upon
connection with a driving circuit, thereby bringing about an undesirable
phenomenon of display defect at present. The phenomenon becomes more
conspicuous as the resistivity of the transparent electrode is lower and
the film thickness of the transparent electrode is greater, and as a
result, the yield of the liquid crystal display reduces.
Recently, the liquid crystal display using the flexible film substrate has
been required to provide more fine and accurate display, high
display-capacity and large display-area, and accordingly, the suppression
of the occurrence of cracks in the transparent electrode is demanded.
As a result of an earnest study for providing a liquid crystal display of
more fine and accurate display, high display-capacity and large
display-area by using a flexible polymer film substrate and with no
occurrence of cracks in the transparent electrode formed on the substrate,
it has been found that a metal-oxide film formed on a polymer film
substrate by means of a specified sputtering method, vacuum vapor
deposition method or ion plating method based on the following knowledges,
which the metal-oxide film has a surface resistivity of not greater than
100 ohm/.quadrature. and is substantially amorphous, is able to dissolve
the foregoing technical subjects:
(1) it is required for the liquid crystal display having a high
display-capacity to reduce the surface resistance for the resistivity of
the transparent conductive film to not greater than 100 ohm/.quadrature.,
but the formation of a liquid crystal display cell becomes extremely
difficult in view of the occurrence of cracks of the transparent
conductive film and the occurrence of curling in the substrate in the case
of reducing the surface resistivity to not greater than 100
ohm/.quadrature.;
(2) as a result of X-ray analysis for ITO films often used generally as the
transparent electrode, the crystallization occurs even in the ITO film
having the surface resistivity of not greater than 100 ohm/.quadrature. on
the flexible polymer film substrate like that in the ITO film on the glass
substrate (on the contrary, the ITO film having the surface resistivity of
200 to 300 ohm/.quadrature. on the flexible polymer film substrate is
substantially amorphous); and
(3) in view of the relationship between the crystallinity in the ITO film
and the bending property (Note (3) described later), curling property
(Note (4) described later) and the etching property (Note (5) described
later) of the ITO film, these three properties are all worsened as the
crystallization is higher. Based on the finding, the present invention has
been attained.
SUMMARY OF THE INVENTION
In an aspect of the present invention, there is provided a liquid crystal
display cell comprising a pair of flexible polymer film substrates having
a transparent electrode on the surface thereof and a liquid crystal
material sandwiched therebetween, the transparent substrate being a
metal-oxide film which has a surface resistivity of not greater than 100
ohm/.quadrature., is substantially amorphous and has a high transmittance.
DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 is a graph representing the result of X-ray diffractiometry of an
ITO film of a liquid crystal display cell according to the present
invention;
FIGS. 2, 3 and 4 are, respectively, graphs representing the result of X-ray
diffractiometry of comparative ITO films; and
FIG. 5 is a graph representing the values of the physical property of ITO
films which are formed while varying the film-forming conditions.
DETAILED DESCRIPTION OF THE INVENTION
A metal-oxide film as a transparent electrode is formed on a flexible
polymer film substrate according to the present invention by using a
specific film-forming method, for example, specified sputtering method,
specified vacuum vapor deposition method or specified ion plating method,
has a surface resistivity of not greater than 100 ohm/.quadrature.,
preferably, from 10 to 80 ohm/.quadrature.; has a substantially amorphous
structure (having not less than 50% of an amorphous portion), preferably a
structure having not less than 70% of the amorphous portion; and is
transparent in a visible region, specifically, with the transmittance to
visible rays (at 550 nm) of not less than 75%, preferably not less than
80%. It is supposed that the worsening of the bending resistance, etc. of
an ITO film observed by reducing the resistivity of the ITO film is
attributable to the progress of crystallization in ITO. In particular, for
reducing the surface resistivity to about 100 ohm/.quadrature. or lower,
it is required to increase the thickness of the ITO film and, since
crystallization proceeds along with the increase of the thickness, the
tendency of worsening the bending resistance, etc. becomes remarkable.
Then, when the bending resistance, etc. of the ITO film having the surface
resistivity of not greater than 100 ohm/.quadrature. and a substantially
amorphous structure as a transparent electrode formed on the flexible
polymer film substrate in the liquid crystal display cell according to the
present invention is examined, it has been confirmed that a satisfactory
bending resistance equal with that of the known ITO films having a
resistivity about from 200 to 300 ohm/.quadrature., which has a
substantially amorphous structure. (refer to FIG. 1). In the present
invention, "substantially amorphous structure" means that there is not
less than 50% of the amorphous portion in the entire film (less than 50%
of crystallinity).
There are specific conditions for forming an ITO film having the surface
resistivity of not greater than 100 ohm/.quadrature. and being
substantially amorphous on the flexible polymer film.
As the film-forming method, specified sputtering method, vacuum vapor
deposition method or ion plating method can be employed and one example of
the film-forming conditions by means of a DC sputtering process is as
shown below.
Mixing ratio between O.sub.2 as a reaction gas and Ar as a carrier gas:
O.sub.2 /Ar partial pressure ratio: 5-20/1,000,
preferably 5-10/1,000
Vacuum degree:
10.sup.-3 -10.sup.-4 Torr
Film-forming rate:
5-13 .ANG./sec, preferably 8-10 .ANG./sec
Sputtering voltage:
200-500 V
Substrate temperature:
not more than 120.degree. C., preferably 40.degree.-100.degree. C.
ITO film thickness:
500-2000 .ANG., preferably 1000-1500 .ANG..
(ITO containing 5 wt % of Sn was used as the target).
FIG. 5 is a diagram showing the result of measurements that represent the
surface resistivity and bending resistance ratio of ITO films obtained by
applying ITO film-forming while varying the ratio of the carrier gas and
the reaction gas in the gases introduced in the forming conditions of the
present invention. If the results are divided into parts above and below
100 ohm/.quadrature. for the surface resistivity, and into parts above and
below 2.0 for the bending resistance ratio, regions A, B, C and D are
obtained. The results are shown in Table 1. Preferred ITO films are those
having the film surface resistivity: not less than 100 ohm/.quadrature.,
the crystallographic nature: amorphous, the bending property (bending
resistance ratio): not greater than 2.0, less curling degree and easy
etching property. Such ITO film is one manufactured in the region B.
TABLE
______________________________________
Region
Region A Region B Region C
Region D
______________________________________
O.sub.2 /Ar
<0.00 0.005-0.010
0.010-0.018
>0.018
partial
pressure
ratio
Surface not less not greater
not greater
not less
resistivity
than than than than
of ITO film
100 ohm/.quadrature.
100 ohm/.quadrature.
100 ohm/.quadrature.
100 ohm/.quadrature.
crystalline
amorphous amorphous crystalline
crystalline
property (amorphous
(amorphous
(amorphous
(amorphous
portion: portion: portion:
portion:
>50%) >50%) <50%) <50%)
Bending above 2.0 below 2.0 above 2.0
above 2.0
property and
below 2.0
Curling small small great great
degree
Etching easy easy difficult
difficult
property
______________________________________
(Note)
(1) Surface resistivity of ITO film:
Measured by 4-probe method using ROLESTA AP (manufactured
by Mitsubishi Yuka Co.)
The film surface resistivity is necessary to be not
greater than 100 ohm/.quadrature..
(2) Cyrstalline property:
Measured by a relaive method.
It is necessary to be substantially amorphous, that is, more
than 50% of the amorphous portion is present (crystallinity
is less than 50%).
(Relative Method)
The diffraction data obtained from the standard specimens
having known crystallinity are indicated while marking "max"
to the data with greater crystallinity and "min" to the data
with smaller crystallinity.
In the calculation for the crystallinity by the relative
method, data within a range in which the measuring angles are
in common with the two kind of standard data and the data
obtained from the test specimens, are used and calculation is
conducted by determining the correlation coefficient between
them. The correlation coefficient is detemined by the
following formula:
C = .SIGMA. (I(.theta.) - Imin(.theta.))/.SIGMA. (Imax(.theta.) -
Imin(.theta.))
(wherein I(.theta.), Imin(.theta.) and Imax(.theta.) represent X-ray
diffraction
intensity from tested specimen, standard specimen with small
degree of crystallinity and standard specimen of great degree
of crystallinity, respectively,
and .SIGMA. represents the sum of the three data within a range
where the angles are in common with each other).
The crystallinity for the test specimen is calculated in
accordance with the following formula by using the correlation
coefficient C:
Xcr = C(Xmax - Xmin) + Xmin
(wherein X represents the degree of crystallinity for the
corresponding specimen).
(3) Bending property:
An electrode made of ITO with 300 .mu.m in width and 100 mm
in length was formed by etching the transparent conductive
film on a PET film and 180.degree. extenal bending was applied for
once by way of cylindrical post of 6 mm.phi. (bending with the
ITO film being situated to the outerside) by using a Gardner
bending tester, and durability was evaluated based on an
inter-terminal resistivity ratio of the specimen before and after
bending test and R.sub.0 represents resistivity before bending
test).
The bending property is preferably not greater than 2.0,
more preferably not greater than 1.5.
(4) Curling degree:
A specimen of 100 mm.sup.2 prepared by forming ITO on a PET
film by means of DC sputtering was left on the flat surface of
glass for 1 min in an atmosphere of 20.degree. C. and 45% RH. Then,
the degree of warp of the specimen was evaluated as the height
(H) from the surface. It was defined as large for .vertline.H.vertline.
.gtoreq. 5 mm
and as small for .vertline.H.vertline. <5 mm, and 0 < H < 5 mm is
preferred.
(5) Etching property:
A line pattern of about 300 .mu.m width was etched at 30.degree. C.
by using an aqueous solution comprising 12N--HCl: an aqueous
36 wt % FeCl.sub.3 solution: purified water = 1:1:2 (vol. ratio).
The etching property was defined as "easy" if the etching time
was within 90 sec and as "difficult" if the time was more than
90 sec.
As the metal oxide for forming the transparent electrode in the present
invention, there can be mentioned, for example, ITO, indium oxide, tin
oxide, zinc oxide and antimony sulfide.
As the flexible polymer film substrate in the present invention, there can
be mentioned, for example, polycarbonate film, polyarylate film, polyester
film, polysulfone film, polyester sulfone film and polyethylene
terephthalate film.
As the liquid crystal material in the present invention, there is no
particular restrictions and those that can be served practically are
usable.
The thickness of the metal oxide film forming the transparent electrode in
the present invention is from 500 to 2000 .ANG., preferably 1000 to 1500
.ANG.. The thickness of the flexible plastic film substrate is from 50 to
500 .mu.m, preferably 75 to 200 .mu.m.
It can be apparent that the ITO film according to the present invention is
substantially amorphous from the comparison of X-ray diffraction diagram
for the ITO film as a transparent electrode on a flexible plastic film
(PET) according to the present invention (FIG. 1); X-ray diffraction
diagram for the crystallized ITO film on glass (FIG. 2); X-ray diffraction
diagram for the ITO film in which crystallization occurs to such an extent
of reducing the surface resistivity to not greater than 100
ohm/.quadrature. on a conventional flexible plastic film (PET) (FIG. 3);
and X-ray diffraction diagram for the existent amorphous ITO film with the
surface resistivity of from 200 to 300 ohm/.quadrature. on a flexible
plastic film (PET) (FIG. 4).
The liquid crystal display cell according to the present invention has the
following advantages.
(1) A transparent conductive film with low resistivity and having excellent
bending resistance, curling and etching properties can be obtained on a
flexible film.
(2) Since the curling property and the bending resistance are excellent in
the step of manufacturing liquid crystal display cells using the flexible
film, there are no troubles in the manufacturing steps and cells of a
large area can be fabricated.
(3) With the improvement for the bending resistance, the connection with a
driving circuit portion is facilitated and the display defect can be
eliminated to improve the yield.
(4) Since the transparent conductive film at low resistivity can be
obtained, it is possible to increase the display capacity and enlarge the
surface area for display.
EXAMPLE
The present invention is explained in more detail in the following
examples; however, it should be recognized that the scope of the present
invention is not restricted to these examples.
EXAMPLE 1
A monoaxially stretched PET film of about 100 .mu.m in thickness was used
as a substrate and an ITO film was formed on the substrate by the
following sputtering conditions to obtain a transparent conductive film
having a film thickness of about 750 .ANG., surface resistivity of 75
ohm/.quadrature. and further, having the following properties:
______________________________________
Bending property: 1.6
Degree of curling: 2-3 mm
Etching property: 50-70 sec
______________________________________
A STN type liquid crystal display cell was manufactured by using the
obtained ITO film and, when the cell was driven at 1/200 duty, excellent
display free from defects was obtained. (Sputtering condition)
______________________________________
(1) Mixing ratio between the
6/1000
reaction gas (O.sub.2) and
the carrier gas (Ar)
(O.sub.2 /Ar partial pressure ratio):
(2) Vacuum degree: 1.5 .times. 10.sup.-3 Torr
(3) Film-forming rate: 8 .ANG./sec
(4) Sputtering voltage:
390 V
(5) Substrate temperature:
60.degree. C.
(6) Target: ITO containing 5 wt % of Sn
______________________________________
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